1. Field of the Invention
The present invention relates to an image forming apparatus and more particularly to an image forming apparatus including a plurality of image forming units.
2. Description of the Related Art
Conventionally, an image forming apparatus is known which includes a plurality of (for example, two) image forming apparatuses coupled with each other to provide a plurality of image forming units. Such an image forming apparatus forms images in succession through the plurality of image forming units to improve the productivity in two-sided image formation processing (Japanese Patent Application Laid-Open No. 2006-47843 and Japanese Patent Application Laid-Open No. 9-54465).
For example, with an image forming apparatus discussed in Japanese Patent Application Laid-Open No. 2006-47843, a first image forming unit forms an image on the first side of a sheet and a second image forming unit forms an image on the second side of the sheet to maintain high productivity in two-sided image formation processing. The image forming apparatus discussed in Japanese Patent Application Laid-Open No. 2006-47843 eliminates the need of providing a sheet discharge processing apparatus for each of the image forming apparatuses coupled, thus saving more space than a case where a sheet discharge processing apparatus is provided for each image forming apparatus.
However, with the conventional image forming apparatus discussed in Japanese Patent Application Laid-Open No. 2006-47843, at the time of one-sided image formation processing during execution of a combined job including one-sided image formation and two-sided image formation, the second image forming unit allows a sheet to pass through without performing image formation thereon. More specifically, a first feed unit feeds a sheet, then the first image forming unit forms an image on the first side of the sheet and conveys it to the second image forming unit via an intermediate conveyance path. Then the second image forming unit allows the sheet to pass through without performing image formation thereon, and discharges the sheet. This operation is not a desirable form since it may shorten the life of the first and second image forming units. Japanese Patent Application Laid-Open No. 9-54465 discusses an image forming apparatus having two printers for printing both side of the sheet. With the image forming apparatus discussed in Japanese Patent Application Laid-Open No. 9-54465, at the time of one-sided image formation one of the two printers is operated. Japanese Patent Application Laid-Open No. 9-54465 does not discuss a combined job including one-sided image formation and two-sided image formation.